Cationic electrodeposition coating is a process by which even substrates complicated in shape or form and details thereof can be successfully coated and by which coating can be conducted automatically and continuously. Therefore, this process has been generally used as a method for providing automotive bodies and like large-sized substrates complicated in shape and form and required to have highly corrosion resistant with an undercoat. As compared with other coating processes, this process is very highly efficient from the paint consumption viewpoint, and hence economical. It has thus come into wide use as an industrial coating process.
Cationic electrodeposition coating is effected by immersing a substrate in a cationic electrodeposition coating composition for said substrate to serve as a cathode, and applying a voltage thereto. In this coating process, paint film deposition is caused by an electrochemical reaction. Upon voltage application, paint emulsion particles migrate to the surface of the substrate and deposit thereon to form a coat film. The coat film deposited shows an insulation performance so that, in the coating process, the electric resistance of the coat film increases proportionally to the increase in film thickness as the thickness of the deposit film on the surface of the substrate increases. As a result, the paint deposition on the relevant site decreases and, instead, paint film deposition begins at a site having no deposit paint film as yet. In this manner, coating with paint emulsion particles proceeds from site to site until completion of the overall coating. In the present specification, such from-site-to-site sequential paint film formation is referred to as throwing power.
Such cationic electrodeposition coating process is generally applied to the undercoating mainly for corrosion resistance. It is therefore necessary that the paint film thickness be not less than a certain specified value in every place even when the substrates have a complex structure. Since, in cationic electrodeposition coating, insulating coat film portions are formed sequentially on the surface of the substrate, as mentioned above, the throwing power is theoretically supposed to be infinite, and uniform coat film formation must be attained in every place of the substrate.
However, the throwing power is not always satisfactory in the prior art cationic electrodeposition processes; uneven film thicknesses tend to result. Thus, for instance, when evaluated for throwing power by the so-called four-sheet box method, the prior art cationic electrodeposition coating compositions tested all failed to show satisfactory throwing power. Even formic acid-neutralized type coating compositions, which are said to show good throwing powers, gave only G/A values around 48%, thus failing to show satisfactory throwing power.
Consequently, uncoated sites remain and it is difficult to form a deposit film with a desired thickness on those sites that are narrow or limited or that are located within a box-shaped structure. A long time is required if coating of details with coat films with a film thickness not less than a specified value is desired. If an increased film thickness is pursued in those parts which the paint can hardly reach, for example small holes and welded portions, the film thickness in those parts that are in contact with a large quantity of paint, for example outside sheets, will become unnecessarily thick, resulting in waste of paint, electric energy and time.
For reducing such waste, it is necessary to increase the throwing power. A presumable cause of reduced throwing power is that ionic groups, hydrated functional groups and the like contained in the paint remain in the coat films formed and serve as charge transfer media, thus reducing the electric resistance of coat films. For realizing high throwing power in cationic electrodeposition coating, it is necessary to eliminate such factor.
However, for making electrodepositable a coating composition for use in cationic electrodeposition coating, it is necessary to design said coating composition to make it water-based and neutralized. As a result, ions are unavoidably contained therein in an amount not lower than a certain level. Therefore, it is difficult to prevent adverse influences of ionic groups remaining in the coat film deposited. Accordingly, the advent of a cationic electrodeposition coating process in which the throwing power is improved and by which coat films can easily be formed in a short time even on parts otherwise hardly coatable, while avoiding unnecessary increases in film thickness on those parts that are readily coatable has been waited for.